1. Field of the Invention
The present invention relates to a fuel cell in which liquid fuel is directly supplied to an anode.
2. Description of the Related Art
A fuel cell is a device that generates electricity from hydrogen and oxygen and achieves highly efficient power generation. Unlike conventional power generation, a fuel cell allows direct power generation that does not require conversion into thermal energy or kinetic energy. As such, even a small-scale fuel cell achieves highly efficient power generation. Other features unique to a fuel cell include less emission of nitrogen compounds, etc. and environmental benefits due to small noise and vibration. As described, a fuel cell is capable of efficiently utilizing chemical energy in fuel and as such environmentally friendly. Fuel cells are envisaged as an energy supply system for the twenty-first century and have gained attention as a promising power generation system that can be used in a variety of applications including space applications, automobiles, mobile appliances and large and small scale power generation. Serious technical efforts are being made to develop practical fuel cells.
Of various types of fuel cells, a solid polymer fuel cell is unique in its low operating temperature and high output density. Recently, direct methanol fuel cells (DMFC) are especially highlighted. In a DMFC, methanol water solution as a fuel is not reformed and is directly supplied to an anode so that electricity is produced by an electrochemical reaction induced between the methanol water solution and oxygen. Reaction products resulting from an electrochemical reaction are carbon dioxide being emitted from an anode and generated water emitted from a cathode (see patent document No. 1). Methanol water solution is richer in energy per unit area than hydrogen. Moreover, it is suitable for storage and poses little danger of explosion. Accordingly, it is expected that methanol water solution will be used in power supplies for automobiles, mobile appliances (cell phones, notebook personal computers, PDAs, MP3 players, digital cameras, electronic dictionaries (books)) and the like.
FIG. 7 is a sectional view showing the schematic structure of a related-art DMFC. The DMFC has a membrane electrolyte assembly provided with a fuel electrode 530 and an air electrode 560 sandwiching an electrolyte membrane 500. The fuel electrode 530 includes an anode catalyst layer 510 and an anode diffusion layer 520. The air electrode 560 includes a cathode catalyst layer 540 and a cathode diffusion layer 550. The fuel electrode 530 is held in place by ribs 572 provided in a casing 570. Similarly, the air electrode 560 is held in place by ribs 582 provided in a casing 580. The illustrated structure is disclosed in patent document No. 1.
[Patent Document No. 1]
    JP 2005-209584
In the related-art DMFCs, nut and bolt clamping is required in order to reduce contact resistance in a diffusion layer, a catalyst layer and an electrolyte membrane. This calls for rigidity of the DMFC components sufficient to withstand clamping pressure. Securing rigidity also requires ribs for holding current collectors in place. As a result, the DMFC has to become more complex and less compact in structure, preventing diffusion of fuel, air and generated products. There are also problems in that size reduction and maintenance are difficult, and the cost is increased. A phenomenon called methanol crossover is known in which methanol flows through an electrolyte membrane along with protons. Methanol crossover causes waste of methanol water solution (liquid fuel), leading to reduction in power generation efficiency. Methanol crossover is more likely to occur when an electrolyte membrane swells. Therefore, suppression of swelling of an electrolyte membrane is a technical challenge to be achieved in order to prevent methanol crossover.
As mentioned before, DMFCs are expected to power portable devices. Therefore, further size reduction and thickness reduction are essential to advance practical applications of DMFCs.